narrow: delegate the narrow spec writing to the transaction

This make it more transactional and will help us to simplify their backup.

The implementation is not great, but it keep the patch simple as this is not the
time for a larger refactoring yet.

mercurial/localrepo.py

@@ -1469,6 +1469,8 b' class localrepository:'
         # post-dirstate-status hooks
         self._postdsstatus = []
 
+        self._pending_narrow_pats = None
+
         # generic mapping between names and nodes
         self.names = namespaces.namespaces()
 
@@ -1799,7 +1801,11 b' class localrepository:'
 
         A tuple of (includes, excludes).
         """
-        return narrowspec.load(self)
+        # the narrow management should probably move into its own object
+        val = self._pending_narrow_pats
+        if val is None:
+            val = narrowspec.load(self)
+        return val
 
     @storecache(narrowspec.FILENAME)
     def _storenarrowmatch(self):

mercurial/narrowspec.py

@@ -5,6 +5,7 b''
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2 or any later version.
 
+import weakref
 
 from .i18n import _
 from .pycompat import getattr
@@ -174,10 +175,40 b' def load(repo):'
 
 
 def save(repo, includepats, excludepats):
+    repo = repo.unfiltered()
+
     validatepatterns(includepats)
     validatepatterns(excludepats)
     spec = format(includepats, excludepats)
-    repo.svfs.write(FILENAME, spec)
+
+    tr = repo.currenttransaction()
+    if tr is None:
+        repo.svfs.write(FILENAME, spec)
+    else:
+        # the roundtrip is sometime different
+        # not taking any chance for now
+        value = parseconfig(repo.ui, spec)
+        reporef = weakref.ref(repo)
+
+        def clean_pending(tr):
+            r = reporef()
+            if r is not None:
+                r._pending_narrow_pats = None
+
+        tr.addpostclose(b'narrow-spec', clean_pending)
+        tr.addabort(b'narrow-spec', clean_pending)
+        repo._pending_narrow_pats = value
+
+        def write_spec(f):
+            f.write(spec)
+
+        tr.addfilegenerator(
+            # XXX think about order at some point
+            b"narrow-spec",
+            (FILENAME,),
+            write_spec,
+            location=b'store',
+        )
 
 
 def copytoworkingcopy(repo):